This invention relates to a double-clad optical fiber and to a process for the production of such a fiber. More specifically, this invention relates to a double-clad optical fiber in which the outer cladding on an end portion of the fiber differs from that on the central portion of the fiber, thereby facilitating polishing or other mechanical manipulation of the fiber.
Double-clad optical fibers comprising a core having an index of refraction n.sub.1, an inner cladding surrounding the core and having an index of refraction n.sub.2, and an outer cladding surrounding the inner cladding and having an index of refraction n.sub.3 (wherein n.sub.1 &gt;n.sub.2 &gt;n.sub.3) are well known and are described, for example, in U.S. Pat. Nos. 4,815,079; 5,268,978; 5,373,576 and 5,418,880. Such double-clad optical fibers are used primarily as fiber lasers, and when so used the core is normally a single-mode core of laser material which is disposed within a multi-mode inner cladding.
The number of materials which are practical for use in the core of such a double-clad optical fiber is small, and accordingly the available values of n.sub.1 are limited; a typical core material is a rare earth-doped silica, having n.sub.1 .about.1.47. The inner cladding is typically substantially pure silica, having n.sub.2 .about.1.46. Furthermore, as is well known to those skilled in the optical fiber art, the numerical aperture of a double-clad fiber depends upon the difference between the refractive indices of the inner and outer claddings (i.e., on the difference between n.sub.2 and n.sub.3) and since a large numerical aperture is desirable to facilitate introduction of light into the fiber, n.sub.3 should be substantially less than n.sub.2 ; in practice, n.sub.3 should be less than about 1.43, and desirably not more than about 1.39. Furthermore, the outer cladding needs to have good adhesion to the inner cladding and low light attenuation to avoid loss of energy from the fiber. Finally, the material of the outer cladding must be such that a uniform thin layer of outer cladding can be formed at high speed upon the inner cladding.
The number of materials which can meet all these requirements for the outer cladding is extremely limited, and considerable research has been devoted to developing outer cladding materials. In practice, the only materials which meet the requirements of refractive index, adhesion, low attenuation and ease of application are radiation-cured polymers derived from highly fluorinated acrylates or similar monomers. For example, U.S. Pat. No. 5,024,507 describes the preparation of a cladding from a photopolymerizable composition comprising an unsubstituted or fluorosubstituted diacrylate monomer; a fluorinated monofunctional acrylate monomer in an amount of from about 2 to about 12 parts by weight per part by weight of the diacrylate monomer; a photoinitiator; and a viscosity modifying agent to increase the viscosity of the composition to about 1000 to about 15000 cP. Upon photocuring with ultra-violet radiation, the composition has a refractive index not greater than about 1.43, and preferably not greater than about 1.40. Similarly, U.S. Pat. Nos. 5,484,822; 5,492,987 and 5,534,558 describe a process, for cladding an optical fiber, in which a photoinitiator monomer, having both a photoinitiating group and an ethylenically unsaturated group, is reacted with a fluorosubstituted monomer having an ethylenically unsaturated group, thereby preparing a copolymer having pendant photoinitiating groups. This copolymer is then mixed with a fluorosubstituted diacrylate, thereby forming a photopolymerizable composition, which is coated on to the optical fiber and exposed to ultraviolet light, thereby curing the photopolymerizable composition to produce the cladding. Preferred claddings of this type can have refractive indices below 1.35. (For convenience, outer claddings produced using the composition of the aforementioned U.S. Pat. Nos. 5,024,507; 5,484,822; 5,492,987 and 5,534,558 will hereinafter be called "Fluoropolymer claddings".)
Although Fluoropolymer claddings meet the requirements of refractive index, adhesion, low attenuation and ease of application to fibers, they are, like most highly fluorinated polymers, soft, having hardness values not greater than about HDD-50, where "HDD" refers to the Hardness Durometer D grade hardness measured in accordance with American Society for Testing and Materials, Standard D2240 (usually abbreviated "ASTM D2240"); all hardness values quoted herein are measured at 23.degree. C. The softness of Fluoropolymer claddings tends to cause problems during mechanical operations on the double-clad fibers. In most uses of such fibers as fiber lasers, it is necessary to provide one or both ends of the fiber with a reflective end surface, i.e., with an optically flat end surface extending perpendicular to the axis of the fiber, or with a similar, closely controlled end surface to allow reliable optical contact with an external mirror. Such an end surface is normally prepared by polishing the end of the fiber. Given the small diameter, of the order of 0.5 mm, and the consequent flexibility of the fiber, it is necessary to confine the fiber in a rigid holder during polishing, and in practice it is convenient to fix the end of the fiber (normally with a strong adhesive) within a capillary tube, with the end of the fiber essentially flush with the end wall of the tube, and to polish the ends of the fiber and the tube simultaneously. Unfortunately, the Fluoropolymer claddings are so soft, and the differences in coefficient of thermal expansion between them and glass capillaries so large, that during such a polishing operation the fiber tends to extrude out of the capillary and it is very difficult to control the exact form of the end surface produced; thus, a good reflective end surface is often not formed. Similar problems may occur during other mechanical operations on the fiber.
It has now been found that the aforementioned problems with Fluoropolymer clad fibers can be reduced or eliminated, without significant adverse effects on the optical properties of the double-clad fiber, by modifying the outer cladding on an end portion of the fiber.